Stabilization Mechanism of Highly Toxic Hexavalent Chromium during Carbothermal Reactions in Lettuce

As a remediation plant for remediating cadmium-contaminated soil, highly toxic Cr-(VI) is enriched in lettuce, making the rational treatment of this biomass a challenge to be solved. In this work, CaO, which is cheap and available, was used as a stabilizer for the pyrolysis of polluted lettuce. Results revealed that the Cr-(VI):Cr ratio in lettuce-derived biochar obtained at 800 °C decreased by 93.6% in comparison to that at 400 °C due to the immobilization effects. Spearman correlation analysis also revealed that Cr-(VI) was negatively correlated with CaO dosage (R ² = -0.81, p ≤ 0.01) and pyrolysis temperature (R ² = -0.86, p ≤ 0.01). Furthermore, Cr-(VI) was reduced to Cr-(V) and Cr-(III) in the presence of CaO during the pyrolysis. XRD analysis and SEM-EDS analysis further confirmed that the Cr-(V) compound refers to Ca₅(CrO₄)₃OH as well as Ca₅(CrO₄)₃Cl. The intensity of the peak of the aromatic ring and heteroaromatic compounds on the surface of biochar presents a significant increase when the CaO dosage was higher than 10%. Furthermore, these redox-active functional groups act as electron donors in the reduction of Cr-(VI). Meanwhile, Cr-(VI) was partially converted preferentially to Ca₂CrO₄Cl in the copresence of CaO and Fe₂O₃, resulting in the immobilization of Cr-(VI). This study clarifies the influence of CaO dosage and temperature on the Cr-(VI) transformation and characteristic of biochar and provides a new strategy for Cr-(VI) remediation of the Cr-contaminated biomass.